This proposal will examine the role of CD5 in mouse B cell development and activation. This molecule is highly expressed on T cells and to a lesser extent on subpopulations of B cells. In the mouse, the expression of CD5 defines the B1a B cell lineage, which appears to fulfill a functional role distinct from the B2 lineage. CD5 expression is also observed predominantly on human B cell lymphomas and leukemias, suggesting a possible role for this molecule in malignant transformation. In this proposal the hypothesis will be tested that CD5 regulates B cell activation and proliferation by two major mechanisms: 1) by the recruitment and activation of signaling molecules; and 2) by actively modulating its level of expression on the cell surface. The investigator has found that the cytoplasmic tail of CD5 associates with Casein Kinase 2 (CK2), in an activation independent manner. This suggests that CD5 bound CK2 may be involved in membrane-proximal signaling events, contrary to current dogma regarding the role of CK2 in regulation of cell activation and proliferation. Signaling via CD5 appears also to be modulated by regulation of levels of cell surface expression. CD5 transgenic mice express high levels (5-10X normal) of CD5 on pro, pre and immature B cells, but levels on mature B cells are much lower. The investigator has found that the cytoplasmic tail of CD5 interacts with the AP50 subunit of the AP2-adaptor complex that links cytoplasmic tails of membrane proteins to clathrin-coated pits. To address the importance of the CD5:CK2 and CD5:AP2 pathways in vivo, the investigator proposes to reconstitute CD5 knockout mice with mutant forms of CD5 that cannot interact with these proteins. The specific Aims of the proposal will involve: 1) examine activation pathways in B cells derived from normal, CD5 transgenic, and CD5 ko mice; and to identify proximal targets of CD5 signaling; 2) To define the in vivo role of the CD5:CK2 interaction by reconstituting CD5 ko mice with a mutant CD5 that cannot bind CK2; 3) To examine the mechanism of CD5 endocytosis via clathrin-coated pits, and to investigate the role of CD5 crosslinking in this process and the fate of internalized CD5; and 4) To define the in vivo role of CD5 endocytosis by complementing CD5 ko mice with a mutant CD5 that cannot interact with the AP50 adaptor molecule.